A large number of compositions are known for removing or reducing the hydrogen sulphide content of and removing mercaptans from natural gas, crude oil, and other hydrocarbon streams. These compositions include those based on alkanolamine and aldehyde reaction products such as described in U.S. Pat. No. 4,978,512 which issued on Dec. 18, 1990 to Quaker Chemical Corporation. These reaction products include triazine compounds, and in particular 1,3,5 tri-(2-hydroxy-ethyl)-hexahydro-S-triazine.
While alkanolamine and aldehyde reaction products such described in U.S. Pat. No. 4,978,512 are effective hydrogen sulphide scavengers, they are known to form solids upon reaction with hydrogen sulphide. In particular, the reaction with hydrogen sulphide forms dithiazine which is not water soluble. Dithiazine forms a separate liquid phase or layer in gas processing equipment. In temperatures of about 20° C. or lower, solid dithiazine crystals form in this layer and precipitate out of solution.
Substantial amounts of dithiazine solid deposit buildup routinely occurs in gas processing equipment, particularly in colder weather applications. These dithiazine deposits are crystalline in nature and are particularly difficult to remove. For example, in bubble tower gas contactors, dithiazine crystals build up in the bottom of the tower and plug the lines through which the spent scavenger chemical is drawn off. Similarly, dithiazine forms a solid crystalline layer in the bottom of the spent scavenger chemical storage tanks located at the well site. Even for inline injection applications, dithiazine crystals tend to accumulate in areas of line restriction such as chokes and in dips or “dead spots” in the gas line. Further, crystallization can occur in bulk truck tanks used to haul away spent product and can cause plugging problems in produced water disposal wells. An enormous amount of effort continues to be expended simply in cleaning out gas processing equipment to remove dithiazine deposits. Often, the only practical solution is to manually chip away the deposits and/or dissolve the dithiazine deposits by steam or hot water. This is an expensive and time consuming process which requires the gas processing equipment to be shut down until treatment is complete.
There have been a number of attempts to improve the performance of triazine based scavengers. For example, there is disclosed in U.S. Pat. No. 5,347,004 and U.S. Pat. No. 5,554,349 (Rivers et al.), the use of a mixture of amines to scavenger hydrogen sulphide from natural gas, The mixture includes reaction products or blends of alkoxyalkylene amines, ammonia and dialkylamines with formaldehyde wherein the reaction product is a hexahydrotriazine having at least one substituent being an alkoxyalkylene group. While Rivers et al. discloses that the amine mixture may be employed without the need for antifreeze or polymerization inhibitors, there is no suggestion that the amine mixture will have any effect on the formation of dithiazine solids.
There is disclosed in PCT international application No. PCT/CA97/00826 (Warrender et al.) a scavenger composition which comprises a reaction product of formaldehyde. with aminoethylpiperazine (AEP) and a second or “enhancing” amine, which can be monoethanolamine. The reaction product is described as reacting with impurities in the natural gas to form a hydrophobic component which forms a second liquid phase so as to solubilize the hydrophobic reaction products to preclude the formation of solids. It would appear from the disclosure, that the solids referred to are not dithiazine solids. The test results in the disclosure indicate that solids formed slowly over time, presumably at room temperature, after the scavenger compositions were subjected to hydrogen sulphide breakthrough tests, i.e., by “overspending” the scavenger. These solids were apparently not observed when a sufficient amount of the enhancing amine was used. It has been found that solids that are produced at room temperature in conventional laboratory hydrogen sulphide breakthrough tests by continuing to overspend the scavenger composition until well after breakthrough are not of the type typically experienced in the field. It is believed that over spending a triazine based scavenger in a laboratory test causes the third ring nitrogen in the triazine molecule to be displaced by sulphur causing the ring to open and the resulting molecule to undergo a slow polymerization type reaction to produce an amorphous solid.
This type of solid is not typically encountered in field applications as the scavenger chemical must be replaced or regenerated at or before breakthrough in order to avoid production of non-merchantable gas. Moreover, dithiazine solids do not usually form at room temperature. The solidification point of pure dithiazine is about 46° C. However, residual methanol (typically present as part of the Formalin component) plus other components in reacted triazine based scavengers mixes with the dithiazine layer and lowers the solidification point below 20° C. Thus, solids formation observed in laboratory breakthrough tests at room temperature are typically not dithiazine. It is very difficult to form a dithiazine crystal in laboratory tests. In field applications, methanol is more readily lost and hence initiation of crystal formation is more apparent. There also may be other impurities in field gas streams or equipment which seed initial crystallization.
Accordingly, while the prior art teaches a number of possible solutions to solids buildup associated with triazine based hydrogen sulphide scavengers, none have effectively addressed the particular problem of dithiazine solids buildup in field applications. There remains a need for a scavenger composition that is effective at removing hydrogen sulphide and which avoids the problems of dithiazine deposit formation characteristic of alkanolamine/aldehyde scavenging products.